The incretins, GIP and GLP-1, are gut hormones secreted in response to nutrients. They have key roles in setting the tone of postprandial metabolism. Pancreatic ?-cells are a principal target of the incretins: about 50% of glucose-stimulated insulin secretion is the result of incretin activity. Incretins have effects beyond ?-cells. For example, GIP enhances insulin-sensitivity of adipocytes, reinforcing a postprandial anabolic metabolism in those cells. Because of the ?pro-insulin? activity of the incretins (i.e., enhanced insulin secretion and action), they are excellent targets to treat insulin resistance. Some of this therapeutic promise has already been met, as drugs that target incretins are currently in use. The effects of incretins on whole body physiology, though complex, are fairly well described. More significant gaps remain in our knowledge of incretin biology at the cellular level, which is especially true for GIP. The objective of this project is to address those gaps by a thorough examination of incretin receptor biology. Incretins signal by G protein-coupled receptors (GPCRs). It is well established that the post-activation trafficking of GPCRs contributes significantly to their biological effects. We have established the tools to study the behavior of the GIP receptor (GIPR) in adipocytes, a physiologically relevant cell type. Using those tools, we have: i) defined the effect of GIP effect on cultured adipocytes; ii) established specialized GIPR trafficking that does not conform to canonical GPCR behavior; and iii) established aberrant intracellular trafficking of a human GIPR coding variant (Q354E) associated with human metabolic alterations. In this application I propose to build on that foundation to further define incretin biology at the cellular level and to extend that knowledge to whole animal studies. We will: 1) Characterize ?-Arrestin2 regulation of GIPR trafficking and control of GIPR enhancement of insulin action in adipocytes. 2) Define trafficking of the incretin receptors (GIPR and GLP-1) in ?-cells, revealing how the behavior/activity of one receptor influences the behavior/activity of the other. 3) Investigate the impact of the GIPR-Q354 on metabolism in studies of mice whose endogenous GIPR has been engineered to model the human GIPR-Q354, which will provide unique insights into the physiological disruptions induced by altered post-activation trafficking of the GIPR. This project addresses significant gaps in knowledge of incretin biology, an enhanced understanding that will inform future efforts to harness the incretin system in the treatment of metabolic diseases.